DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

orbit around the sun, are stable points in a three
body system (sun – planet – asteriod). For the
case of the planet Jupiter, a number of asteriods
have been found in this region and are all named
after heroes of the Trojan wars.
Trojans asteroids Asteroids located near
Jupiter’s Lagrange points (60 ◦ ahead and behind
Jupiter in its orbit).
tropical cyclone A large low pressure system
that originates over the tropical oceans; including
tropical depressions, tropical storms,
and hurricanes (cyclones, typhoons), with winds
sometimes up to 300 km/h.
tropical instability waves Oceanic disturbances
of meridional scale of 500 km, zonal
wavelength of 1000 km and a period of 30 days
that travel westward along 2-3 ◦ N in the equatorial
Atlantic and Pacific Oceans. Instabilities of
velocity shears among equatorial currents are
their cause. They start to grow in June and
decay toward the end of a year, in correlation
with the intensity of equatorial currents. They
are well visible from satellite infrared images
as they cause meanders of a strong sea surface
temperature (SST) front between the equator
and 5 ◦ N. Often called the Legeckis waves after
the discoverer of the waves’ signature in SST.
These wave-induced SST variations excite atmospheric
waves of significant amplitudes in the
marine boundary layer.
tropical month See month.
tropical storm A storm originating in the
tropics; particularly severe tropical storms become
hurricanes (sustained wind speeds >
120 km/h).
tropical year See year.
Tropic of Cancer The parallel of latitude
23 ◦ 27 ′ N. The sun lies in the zenith (overhead
at noon) at this latitude, at the summer solstice,
around June 22. North of this latitude, the sun
is always south of directly overhead.
Tropic of Capricorn The parallel of latitude
23 ◦ 27 ′ S. The sun lies in the zenith (overhead
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trough
at noon) at this latitude, at the winter solstice,
around December 22. South of this latitude, the
sun is always north of directly overhead.
tropopause The boundary layer between the
troposphere and the stratosphere. In the troposphere,
air temperature generally decreases
with height, and in the lower parts of the stratosphere,
air temperature remains constant or increases
(0.1 to 0.2 ◦ C/100 m). The altitude of
the tropopause varies with the variations of sea
— surface temperature, season, latitude, and
weather systems, such as the passage of cyclones
and anti-cyclones. It has its highest
height (about 17 to 18 km) over the equator
and lowest (about 6 to 8 km, average 10 to
12 km) over the poles. There are two different
types of tropopause. One is the pole-region
tropopause with height lower than 150 hPa; another
is the tropical-subtropical tropopause with
height higher than 150 hPa. Because of this
distinct height difference, when these two different
types of air mass systems approach, the
tropopause will appear as a broken phenomenon.
Under such a broken tropopause, there is
often a high level jet which is related to the front.
troposphere The layer of the atmosphere
closest to the planet’s surface. The troposphere
is characterized by a decrease in temperature
with altitude, caused by a decrease in the amount
of warming due to the Greenhouse Effect with
altitude. Hadley Cell circulation and the Greenhouse
Effect dominate the temperature profile
and weather patterns within the troposphere. On
Earth, the troposphere extends to about 25 km
above the surface. The exact height varies with
latitude and season. The boundary between the
troposphere and the stratosphere is called the
tropopause. The troposphere is also often called
the lower atmosphere.
trough A long, relatively wide trough running
on sea bottoms, whose maximum depth of
the water is less than 6000 m. This definition
is applied irrespective of the origin of troughs.
There are some troughs whose structure and origin
are basically the same as those of trenches.
Such an example is the Nankai trough situated
on the Pacific side of the Japanese islands. For
such a trough, as is the case for a trench, seismic
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